A grain-oriented electrical steel sheet is mainly utilized as an iron core of a transformer and required to exhibit superior magnetization characteristics, e.g. low iron loss in particular.
In this regard, it is important to highly accumulate secondary recrystallized grains of a steel sheet in (110)[001] orientation, i.e. what is called “Goss orientation”, and to reduce impurities in a product steel sheet. However, there are restrictions on controlling crystal grain orientations and reducing impurities, in view of production cost. Accordingly, there has been developed a technique of introducing non-uniformity (strain) into a surface of a steel sheet by physical means to subdivide width of magnetic domains to reduce iron loss, i.e., magnetic domain refinement technique.
For example, JP S57-2252 A (PTL 1) proposes a technique of irradiating a steel sheet as a finished product with a laser beam to introduce linear high-dislocation density regions into a surface layer of the steel sheet, thereby narrowing magnetic domain widths and reducing iron loss of the steel sheet. The magnetic domain refinement technique using laser-beam irradiation of PTL 1 was improved thereafter (see JP 2006-117964 A (PTL 2), JP H10-204533 A (PTL 3), and JP H11-279645 A (PTL 4)), so that a grain-oriented electrical steel sheet having good iron loss properties can be obtained.
A device for irradiating a laser beam as described above needs to have a function of linearly irradiating a laser beam in the width direction (direction orthogonal to the rolling direction) of the steel sheet. For example, JP S61-48528 A (PTL 5) discloses a method of using an oscillating mirror, and JP S61-203421 A (PTL 6) discloses a method of using a rotary polygon mirror, each of which is a method for scanning a laser beam in the width direction of a steel sheet under specific conditions.
Meanwhile, JP H06-072266 B (PTL 7) proposes a technology of controlling the width of magnetic domains through irradiation of an electron beam. According to this method, which reduces iron loss through irradiation of an electron beam, the electron beam can be scanned at high speed through magnetic field control, which means that the method involves no mechanical moving element that is employed otherwise in an optical scanning mechanism for a laser beam. Therefore, the method is particularly advantageous in continuously irradiating an electron beam at high speed onto a continuous strip having a wide width of 1 m or more.